PTK7 and clear cell renal carcinoma: Recent studies have reported that PTK7 has overexpressed levels in various solid tumors, including colon cancer, lung adenocarcinoma, acute myelogenous leukemia, and gastric cancer [15–18], and abnormally high expression of PTK7 often indicated poor prognosis for patients; however, the PTK7 expression level in certain tumors, such as clear cell renal cell carcinoma, melanoma, and epithelial ovarian cancer, was relatively low [29–31].